I have a luggage rack on the back of my bike, and I just bungee down a milk crate (the kind that 4x gallons are delivered in) when I take things around. They're light, and between a bungee and the spring on the rack it holds everything tight.

I don't have a picture, but it was one like this, but square. Plenty of holes to bungee down on! I'm surprised you haven't seen a moped or cyclist doing that yet, maybe it's a Spokane thing? I see them all the time! =)

I actually used the old original bike rack that was on that old farm cruser I revamped into....Da Da Da Daaaaaaaa..."ZAT'S MOTOR GOOSE" Then I went down to the Army surplus store and found a slue of packs and Carry alls and tool bags and ammo bags and packs. ITS A SMORGASBOARD OF BAGGERY! And cheep too! $12.00 a bag! Well I bought two bags where the top flaps over then buttons on each side of the bag corners. I believe they held marker lights, Got about a 5" U.S. stencled in the center of the bag. Looks cool as H$ll. Then velcro'd the tops together and hung the buggers over the rack like THEM THAR COWBOYS DO! So when I figure out how to get a picture from my Iphone to here ill post pic's of the old motor Goose.

Don't know if this link works but I put like a milk carton on the back rack, put in a gas tank, lined it with an old sign and made a cover. Nobody even knows what's in it. I put white pieces of reflective on the outside and a few flashing bike lights on the back for extra viewing when riding in the rain and early morning hours.

I am in the proccess of mounting duel large rectangular milk crates to the seat mounted heavy duty rear rack of my bicycle. I want to mount the racks so they ride as low as possible to leave open backpack space above the racks. Im posting pics soon